Recently, attention is focused on hybrid vehicles and electric vehicles as vehicles taking into account environmental issues. A hybrid vehicle includes, in addition to a conventional engine, a direct current power source, an inverter, and a rotating electric machine (motor) driven by the inverter as the power source. In addition to achieving the power source by driving the engine, the direct current voltage from the direct current power source is converted into alternating voltage by the inverter, and the converted alternating voltage is used to rotate the motor to achieve power.
An electric vehicle includes a direct current power source, an inverter, and a motor driven by the inverter as the power source.
In such a hybrid vehicle or electric vehicle, the motor is driven in a relatively wide range of rotation from low speed to high speed. There was a problem that the electromagnetic noise generated during driving will become so great that the rider in the vehicle will be disturbed by the noise. Particularly, the electromagnetic noise of harmonics in the range from an idling state where the engine rotational speed is low to the cruising region is humanly audible as annoying noise, differing in frequency from the ground noise caused by the engine and auxiliary machine.
Recently, it has become apparent that this electromagnetic noise of harmonics is greatly affected by the electromagnetic excitation of 6f generated during motor operation. This “6f” implies six times the basic frequency f of the alternating current supplied to the motor.
Japanese Patent Laying-Open No. 2003-348781 discloses a method of reducing such 6f electromagnetic excitation of the motor. There is provided an AC rotating electric machine for a vehicle formed of a stator including a stator iron core (stator core) having two slots per each pole and each phase, and a 3-phase stator coil wound around the stator core, and a rotor provided rotatable at the inner side of the stator, wherein first and second stator coils are connected in series, each having the X-phase coil, Y-phase coil and Z-phase coil constituting the aforementioned 3-phase stator coil wound at adjacent slot groups, and the slots are formed at an irregular pitch of alternate electrical angles of α° and (60−α)° (α≠30°).
According to this configuration, the 6f electromagnetic excitation that was an issue in a conventional rotating electric machine having two slots formed per each pole and each phase at an equal pitch of the electrical angle of 30° can be reduced by optimizing α°. As a result, the electromagnetic noise and vibration caused by the 6f electromagnetic excitation are reduced.
However, the AC rotating electric machine for a vehicle disclosed in Japanese Patent Laying-Open No. 2003-348781 encounters the problem that the 6f electromagnetic excitation cannot be reliably reduced due to the uneven magnetic flux generated at the stator iron core among the plurality of stator teeth in response to the supply of the alternating current to the wound 3-phase stator coil.
Namely, in the AC rotating electric machine of Japanese Patent Laying-Open No. 2003-348781, each of the 3-phase stator coil has series-connected first and second stator coils wound at adjacent slot groups. Specifically, when there are 96 magnetic poles in the rotor, the fourth, tenth, sixteenth, . . . ninety-fourth slots constitute the first slot group, whereas the fifth, eleventh, seventeenth, . . . ninety-fifth slots constitute the second slot group. The first slot group and the second slot group receive the X-phase coil.
Therefore, there is an imbalance in the number of windings of the stator coil between the stator teeth corresponding to the first slot group, having the first stator coil wound, and the stator teeth corresponding to the second slot group, having the second stator coil wound. The electromagnetic excitation applied to the teeth by supply of the alternating current to the stator coil will be out of balance between the teeth. This electromagnetic excitation is divided into a component that causes the stator core to vibrate in the radial direction and a component that causes the stator core to vibrate in the circumferential direction. The radial vibration occurring at the stator core due to the imbalance of the component in the radial direction causes the air in contact with the outer circumferential face or inner circumferential face of the stator core to vibrate, leading to the generation of a large electromagnetic noise.
The present invention is directed to solve such a problem. An object of the present invention is to provide a rotating electric machine that suppresses stator vibration to allow reduction in electromagnetic noise.